Box section is known from a very wide range of applications, components of steel-frame architectural structures, machine frames, crane booms, and beams and frames in shipping vehicles for example.
Box section is usually rolled hot or cold from round tubing and features a high load-bearing capacity accompanied by relatively light weight.
Except for slightly thicker areas at the edges, the flanges, or horizontal walls, and the webs, or vertical walls, of such structural section are uniformly thick and accordingly distribute their mass uniformly over the periphery.
When, however, such structural section is employed for tension or compression members or is subject to bending forces in relation to powerful torsion, one drawback of uniformly thin walls, specifically that the section can essentially be supported only by edge elongation, becomes evident.
This drawback in turn considerably affects overall height and restricts the design potential when for example such section is employed for truck axles or as telescoping crane booms.
When, however, increasing bending stress must be accommodated in conjunction with the maximal possible resistance to torsion, it will become necessary not only to elongate the edges and possibly thicken the whole wall uniformly, but also to redistribute the mass around the periphery of the section to ensure that the varyingly thick walls of the flanges and webs will produce an optimal local equatorial moment of inertia in accordance with Steiner's displacement.
The process of manufacturing section of this type is, however, much more complicated than the aforesaid rolling.
Section of this type is usually composed of joined-together sheet metal, with the flanges and webs made of different thicknesses.
Another approach utilizes two L-shaped sections, with the base of each L being thicker than the riser, joined together laterally reversed into box section.
One drawback of both these approaches is that they require a series of several relatively complicated production steps. The L section for example must first be hot-rolled and stretched, subsequent to which two lengths are positioned together laterally reversed and welded with two diametrically opposed longitudinal seams.
The hereby conventional submerged-arc welding proceeds at approximately two or three meters a minute, permanently decelerates production, and necessitates subsequent heat-treatment of the seam to ameliorate undesired alterations in the joint and welding tensions due to the heat of welding and to uncontrolled cooling.
Another method of manufacturing box section with walls that differ in thickness around the periphery is disclosed in European Patent 0 084 799 B1. This section is employed for interiorly cooled tracks for continuous casting and is also rolled from a round tube.
The thickness of the walls is varied in one embodiment disclosed in the European patent either by grinding down one face of a length of finished box section or track to make the wall thinner or by symmetrically rolling tubing with walls that already differ in thickness.
The variation in wall thickness is primarily intended to improve heat transmission for particular applications.
The drawback of the aforesaid grinding process is the expense dictated by the high level of investment in machinery and by the time involved as well as by the large amount of waste that occurs.
The alternative that begins with tubing that already has thicker wall areas on the other hand also involves serious production-technology drawbacks in that on the one hand it is very difficult to manufacture tubing with uniformly symmetrical thicker wall areas and without eccentricity over its total length and in that on the other it is extremely difficult to roll such tubing into rectangular box section and position the thicker areas accurately, whereby the flow of mass is impossible to control during the rolling.
Another method of manufacturing hollow structures with walls of non-uniform thickness is disclosed in German Patent 843 834. Here the generally round tubing with a uniformly thick wall is shaped by hot drawing or hot extrusion. The starting tubing is heated irregularly along its circumference before being drawn to obtain lower temperatures in the areas subjected to high tension during the drawing process than in those subjected to compression.
This approach prevents undesirable constrictions in the wall, at the corners of drawn box section for example, and also makes it possible to produce hollow section with walls of non-uniform thickness. The latter results from a more powerful heating of the areas, the thinner areas in this case, that is, that are more powerfully affected in terms of the final shape, when the hot drawing employs a core.
Although this method does allow control of the wall thickness along the periphery, it does entail the drawbacks of being time-consuming and complicated in that the original tubing must first be completely drawn over the core and then pivoted up in front of the die along with the core and drawn or extruded, subsequent to which the core must be extracted and pivoted back into the lifting position. The use of a core also limits the method to length-by-length manufacture, and it cannot be extended to the continuous production of such section.
The core is also difficult to support, especially when larger section is being manufactured, and all the tracks and pivoting mechanisms are complicated to build and inefficient to operate.
All known methods are also impractical in that they are impossible to integrate into high-speed plants for manufacturing standard section on an industrial scale.
Both the joining and machining and the hot drawing over a core dictate how rapidly a continuous-operation plant can operate and accordingly represent bottlenecks.